Present invention relates to an image processing apparatus and method for inputting an original image and copying the input image on a predetermined recording medium.
As an image processing apparatus of this type, an apparatus using the ink-jet method as its image recording mechanism has been provided.
Generally, binary image recording involves the steps of determining the status of a dot "black or white", which means discharging/not-discharging an ink, and representing a whole image with a mass of such black/white dots. Recent improvements in this technique have enabled half-tone image recording.
Various printing methods have been introduced as multi-level (half tone, gray-scale) image printing methods, e.g., the error diffusion method and the dither method. These methods basically use a plurality of minute pixels for representing one printing pixel. To form a gradation image, the number of minute pixels is changed to vary the area of one printing pixel.
One printing pixel is formed by repeating printing processing plural times. In contrast to the method for printing one printing with by one of two status (binary image printing method), the number of density levels of the multi-level printing increases in accordance with the number of printing processings, which allows subtle gradation expression.
Further, in order to improve printing speed in the multi-level image printing method, a multi-nozzle construction, i.e., the construction of a printing head comprising a plurality of nozzles (discharging elements, discharging portions) is employed.
In this case, small difference in ink discharge amount (volume) between the nozzles and differences in the shape of nozzles cause density differences between the nozzles. To reduce variations caused by these density differences diffusing the density difference has been considered. The technique makes the density variations unevenness visually inconspicuous and forms an excellent image. A detailed description of the technique will be given below with reference to FIG. 12 which shows the relation between a multi-nozzle head (here the number of the nozzles is thirty) and pixels to be recorded.
In FIG. 12, the printing head is divided into three parts A to C. One printing pixel is represented by a density value 0 to 3 (four-level value) which is attained by actually discharging from zero to three ink drops.
Upon recording, nozzles 1 to 10 perform a first stage of image recording; nozzles 11 to 20, a second stage of recording which is overstrike (overlap) printing on the first recorded image; and nozzles 21 to 30, a third stage of recording which is overstrike printing on the second recorded image. It should be noted that nozzles which discharge an ink drop are sequentially allotted in respective printing stages.
The recording is performed with the three-part printing head, repeating to convey a recording medium (a recording sheet) by 1/3 length of the recording width (10 dot lines) in a longitudinal direction at each scanning of the printing head.
The multi-level printing is performed in this manner. In FIG. 12, one printing pixel is represented by an area for three ink drops, which simplifies the explanation, however, in actual printing, three ink drops are discharged at the same position as overstrike recording.
However, a copying machine having the above printing mechanism has to repeat read-scanning three times while shifting the recording medium by 1/3 of the recording width for the second and third scannings to form a complete printing pixel, which takes a long period. In addition, image data density obtained in the second reading may be different from that obtained in the initial reading due to noise which occurs during the scanning, thus causing subtle difference in density levels between the original image and the output image. In a case where overstrike printing, as a second printing by the nozzles 11 to 20 from the second scanning, is made upon a printed image by the nozzles 1 to 10 from the first scanning, the second image data for the second printing is not necessarily the same as the first image data.